This invention relates to a memory control unit in a display apparatus of the raster scan type, and more particularly to a control system for controlling a refresh memory which stores data to be displayed on a CRT (cathode-ray tube) of the TV scan type.
In the art of digital data display, a refresh memory is generally combined with a CRT for displaying, for example, digitized characters on the display screen of the CRT. Such digital data to be displayed is stored in the refresh memory and is successively read out from the refresh memory in synchronism with the raster scanning of the display screen of the CRT. On the basis of the digital data read out from the refresh memory, a character generator generates character patterns which are entrained on the scanning beam to be displayed on the display screen of the CRT.
Two methods as described below are commonly known and put into practice for the control of the reading out of data stored in such a refresh memory to be displayed on the display screen of a CRT.
In one of these two prior art methods, the refresh memory, from which data to be displayed is read out, functions completely as a read-only memory during the period of character display on the display screen of the CRT. According to this first method, a request for access to the refresh memory from the logic circuit controlling the CRT is not accepted until the end of the refresh time, since the refresh memory is completely occupied for refreshing the display on the display screen of the CRT during the display period. This results inevitably in an extreme reduction of the processing performance of the system during this period. A steal of the refresh cycle during the display period in an attempt to improve the processing performance will result in an improper display during that cycle, tending to provide a flickering display.
The second method is disclosed in, for example, U.S. Pat. No. 3,701,988. In accordance with this U.S. patent, one row portion of data to be displayed is read out from the refresh memory to be stored in a buffer memory during the period of an inter-row space in which no characters are displayed, so that such data can then be read out from the buffer memory to be displayed on the display screen of a CRT. The technical idea of this patent is advantageous in that the provision of the buffer memory storing one row portion of data can shorten the period of time during which the refresh memory is occupied for the reading purpose, so that the processing performance of the system can be improved. The second method is further advantageous in that the refresh memory need not be operated at a high speed since the data can be read out from the refresh memory during the period of an inter-row space. However, this second method is not applicable to a system which is designed to additionally display, for example, graphic characters and/or rulings which extend across or appear in the inter-row space as well. Although a system using a CRT of the full raster scan type to exhibit an additional function of displaying rulings or the like in the inter-row space is widely employed at present, the technical idea of the second method is not applicable to such a system, since data to be displayed is read out from the refresh memory during the period of the inter-row space and no display is provided during this period according to the principle of the second method.